The increase, at constant cost, of computing power now allows the development on an industrial scale of automatic inspection means designed to assist the operators responsible for visually inspecting the tires. These means largely make use of image-processing techniques the performance of which, in terms of speed of analysis and of definition, depends largely on the computing power used.
The methods employed to carry out these processes usually consist in comparing an image in two or preferably three dimensions of the surface of the tire to be inspected with a reference image in two and preferably three dimensions of the surface of said tire. Accordingly, an effort is made to match the image or the surface of the tire to be inspected and the image or the reference surface, for example by superposing them, and the manufacturing anomalies are determined by analysing the differences between the two images or the two surfaces.
A method of this type is described as an example in publication U.S. Pat. No. 5,715,166 which describes the transformations to be carried out in order to match a reference surface with a three-dimensional image of a given object by using transformation functions such as rotational or sliding movements. This method is applied with good results when the aim is to match non-deformable solid objects such as metal parts considered in this instance to be infinitely rigid.
In the case of the tire, the reference image of the surface may be obtained from digital data originating from the design of the tire or, more commonly, from the digital data used to describe and to manufacture the curing mould, said mould itself being intended to give its definitive shape to said tire.
The three-dimensional image of the surface of the tire may be obtained, in a known manner, with the aid of a camera capable of determining the three-dimensional relief of the surface of the tire.
However, it is observed that the tire taken out of the mould does not correspond exactly with the negative image of the mould in which the moulding and curing operation took place, because of the elastic nature of the materials that it is made of.
Specifically, the tire deforms as soon as it comes out of the curing press because of the thermal retractions of the materials when they cool. Moreover, during mounting and inflation, the reinforcing plies take their final position and the equilibrium curve of the inflated tire does not necessarily correspond to the shape of the tire given by the curing mould.
It is therefore necessary to first adjust the image of the reference surface and of the acquired image of the surface of the tire to be inspected, in order to match the two surfaces for the purpose of drawing pertinent information on the conformity of the tire originating from manufacture.
One method of this type is described, as an example, in publication EP 1 750 089. According to this publication, the three-dimensional representation of the surface of the tire to be inspected and the three-dimensional representation of the reference corresponding thereto, are divided into surface portions of reduced dimensions, corresponding substantially to the surface of a marking element such as a letter or a set of letters. Still according to the method in question, the computing algorithms are adapted in order to slide the surface portions, of the reference surface and of the surface to be inspected, one over the other in order to determine the optimum match between the contours of the reliefs of the two surfaces. Once this local adjustment has been carried out, and for the surface portion in question, the two surface portions are compared with one another in order to determine the degree of conformity of the tire to be inspected with a reference.
Although the algorithms described in this publication have the advantage of dispensing, in a large measure, with the position differences between the model and the real tire described above, and the mounting and inflation differences from one casing to another, they are close to those described in publication U.S. Pat. No. 5,715,166 in that they assimilate the surface portions to rigid elements.
Nevertheless a lack of robustness is observed in the application of this method, in particular, when the surface of the tire to be inspected has considerable anomalies such as a total absence of a marking element, or the presence of a foreign element in the marking in relief, which anomalies have the effect of disrupting the algorithm for matching the acquired image and the reference image in this surface portion. Moreover, this method gives poor results in smooth surfaces that do not have any marking and that may also comprise moulding anomalies.